A method including an occlusion period at the start of inspiration to measure the (negative) inspiratory pressure at the end of a 0.1 second occlusion period is described in R. Kuhlen et al., “A New Method For P 0.1 Measurement Using Standard Respiratory Equipment”, Intensive Care Med., 1995 July 21: 545-546.
Proportional assist ventilation (PAV/PPS) is a recent mode of ventilatory assistance. In PAV/PPS mode, the ventilator (also known as respirator) generates pressure at the airway opening in proportion to inspiratory flow (flow assist: FA) and to volume (volume assist: VA), both produced by the contraction of the patient's inspiratory muscles. In PAV/PPS mode, the ventilator operates as a servo-system aimed to reduce selectively the elastic and resistive inspiratory workload. Theoretically, the resistance and elastance of the respiratory system should be known exactly to set adequate levels of PAV/PPS. This implies the use of invasive measurements in spontaneously breathing patients, and sedation or sedation/paralysis in mechanically ventilated patients. The lack of any easy technique to measure the patient respiratory mechanics has been recognized as a major problem for the widespread application of PAV/PPS in the clinical environment.
Recently, it has been suggested by Younes et al. (“A Method for Non-invasive Determination of Inspiratory Resistance During Proportional Assist Ventilation”, Am. J. Respir. Crit. Care Med. 2001; 163: 829-839) that simple maneuvers performed non-invasively during PAV/PPS, might be useful in estimating inspiratory resistance and passive elastance of the respiratory system, see Younes et al. “A Method for Measuring Passive Elastance During Proportional Assist Ventilation”, Am. J. Respir. Crit. Care Med. 2001; 164: 50-60. In particular, inspiratory resistance may be computed performing repeated brief reductions in airway pressure Paw in the early part of the inflation phase. Airway pressure Paw, flow V′ and inspiratory volume V are measured at the beginning of the pulse (T0), at the trough of the pulse (T1), and at a point 0.1 seconds before T0 (T1). Equations of motion of the form Pmus+Paw=V′·K1+V′2·K2+V·E are generated for the data at the three time points (E=elastance, K1 and K2 are the so-called Rohrer's constants). These three equations can be solved for K1 and K2 if it is arranged that the pulse has an appropriate configuration and timing, and if it is assumed that ΔPmus/ΔT is constant for the brief pulse period. This method implies a lot of assumptions and simplifications as well as potential sources of error. Among the latter, the errors related to the extrapolation of the Pmus trajectory are potentially the most serious, particularly when respiratory drive and, hence, the rate of rise of Pmus are high. As a matter of fact, the rate of growth of Pmus during the rising phase in humans is not constant. The passive elastance can be estimated during PAV/PPS by performing a short (0.25 second) end-inspiratory occlusion (EIO), see the second of the above-referenced articles. In these articles it is suggested that Pmus declines to 0 during the end-inspiratory occlusion (EIO), a fact that allows Paw to reach a plateau. The value of Pmus at the plateau should reflect passive recoil at the prevailing volume. However, also in this case, some problems can limit the accuracy of the measurement. Among them are possible patient reactions to the occlusion or a prolonged Pmus decay, and the fact that passive recoil during EIO includes intrinsic positive end expiratory pressure (PEEPi) that has to be taken into account. A further disadvantage of the described method is the measurement of the resistance in the expiration phase, whereas the resistance value is needed for PAV in the inspiration phase. Occlusion (up to 300 ms) at the end of inspiration is rather disturbing for the patients.